Note 6 - Stockholders' Equity, Stock Plans and Stock-based Compensation Expense - Restricted Stock Units Activity (Details) - Restricted Stock Units (RSUs) [Member] - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Outstanding, number of shares (in shares)		510,587		445,317		371,630
Outstanding, weighted-average grant date fair value (in dollars per share)		$ 24.88		$ 11.15		$ 12.39
Outstanding, aggregate intrinsic value	[1]	$ 8,072		$ 23,155		$ 7,726		$ 4,753
Granted, number of shares (in shares)		265,124		412,208		480,191
Granted, weighted-average grant date fair value (in dollars per share)		$ 44.57		$ 28.74		$ 10.80
Vested, number of shares (in shares)	[2]	(231,515)		(224,799)		(172,990)
Vested, weighted-average grant date fair value (in dollars per share)	[2]	$ 21.10		$ 10.91		$ 12.56
Vested, aggregate fair value	[2],[4]	$ 9,483	[3]	$ 5,168	[5]	$ 1,906	[6]
Forfeited, number of shares (in shares)		(69,905)		(122,139)		(233,514)
Forfeited, weighted-average grant date fair value (in dollars per share)		$ 20.01		$ 13.56		$ 11.36
Outstanding, number of shares (in shares)		474,291		510,587		445,317
Outstanding, weighted-average grant date fair value (in dollars per share)		$ 38.44		$ 24.88		$ 11.15

[1] Based on the closing stock price of the Company's stock of $ 17.02 on December 31, 2018, $45.35 on December 31, 2017, $17.35 on December 31, 2016 and $12.79 on December 30, 2015.

[2] The number of restricted stock units vested includes shares that the Company withheld on behalf of the employees to satisfy the statutory tax withholding requirements.

[3] On the grant date, the fair value for these vested awards was $4.9 million.

[4] Represents the value of the Company's stock on the date that the restricted stock units and performance stock units vest.

[5] On the grant date, the fair value for these vested awards was $2.5 million.

[6] On the grant date, the fair value for these vested awards was $2.2 million.